The present invention relates to ground-boring apparatus and a ground-boring method suited for the all-casing method.
Conventionally, cast-in-place concrete pile technology has been widely used for making foundation piles at the construction site of a building, for example, in which each concrete pile is made by boring a hole in the ground, inserting a cage made of steel reinforcing bars into the hole, and pouring concrete into the hole. Various methods are known in the conventional cast-in-place concrete pile technology, including the earth drill method and all-casing method.
The earth drill method is a method of making a hole in the ground using a cylindrical rotary bucket, or using a soil stabilizer to protect the wall of the hole. This method is suited for boring relatively hard ground mainly containing clay, for example.
On the other hand, the all-casing method is a method of making a hole in the ground by driving a casing tube into the ground and digging inside the casing tube. This method allows boring even soft ground, such as reclaimed land. Since a plurality of casing tubes can be driven one on top of another, it is possible to make a long pile which will reach a deep bearing stratum. Moreover, since this method enables boring operation to be done even when there is an obstacle underground, it is frequently used in recent years.
A commonly used technique in the all-casing method for inserting a casing tube into the ground is to drive the casing tube by pressure while turning it about its vertical axis using an all-round rotary boring machine. In all-casing boring operation, it is necessary to dig the ground inside the casing tube and remove soil from the inside of the casing tube. A hammer grab has conventionally been used as a tool for doing this task.
The hammer grab, however, has a problem that its digging and soil-discharging efficiency is rather poor, because it uses a pair of claw members which is operated in a narrow space within the casing tube to dig and pick up soil. The hammer grab also has a problem that it produces a high level of noise as it is repeatedly dropped during the boring operation. Furthermore, since the hammer grab cannot level off the bottom of a hole, a cage which will serve as a core of a pile can not be set in a stable position, leading to a possibility of variations in the strength of finished concrete piles. Moreover, when slime sets at the bottom of the hole, it is necessary to remove it by using a pump, resulting in an increase in the number of processes.
Under these circumstances, a new drilling method is going to be put into practical use today, in which, instead of a hammer grab, a boring screw head is inserted into a casing tube to dig and remove soil at the same time. The boring screw head is raised and lowered along a leader which serves as a guide suspended in a vertical position from a base machine, so that boring operation is performed with the base machine held close to an all-round rotary boring machine in this drilling method. This new drilling method serves to dramatically improve the efficiency of digging and removing soil compared to the aforementioned hammer grab method.
This drilling method, however, has a problem that it can not be used for making a hole if there is an unleveled area or obstacle between the base machine and a boring point, because the base machine must be located close to the boring point where the all-round rotary boring machine is installed. Furthermore, as the distance between the base machine and the boring point can not be made so large, the diameter of the all-round rotary boring machine is limited, making it impossible to bore a hole having a large diameter as a consequence.
Since the vertically positioning accuracy of the boring screw head is affected by the horizontally positioning accuracy of the base machine, it is necessary to level off an area of the ground where the base machine is installed to achieve its highly accurate horizontal position. In addition, since the base machine is dedicated exclusively to boring operation, it is necessary to move it away and bring in an auxiliary crane as the need arises to perform hoisting operation, such as when handling a cage or inserting an additional casing tube.
The present invention has been made in view of the aforementioned problems of the conventional cast-in-place concrete pile technology. Accordingly, it is an object of the invention to provide ground-boring apparatus and a ground-boring method which make it possible to efficiently bore a hole of a desired diameter regardless of ground conditions of a boring site without the need of a dedicated boring machine and thereby reduce the number of processes required for boring operation.
According to an aspect of the invention, a ground-boring apparatus is adapted for boring the ground within a casing tube and removing soil from the bored hole. The casing tube is driven into the ground by a casing tube pusher to prevent the ground from collapsing into a bored hole. The apparatus comprises an extensible telescopic cylinder to be suspended by a movable crane, a boring tool attached to the lower end of the telescopic cylinder, a supporting frame unit to be placed on the casing tube for holding the telescopic cylinder rotatably about its vertical axis, a driver provided on the supporting frame unit for turning the telescopic cylinder about its vertical axis, and an interlock device for joining the supporting frame unit and the casing tube to thereby ensure the rotation of the telescopic cylinder counteracting a reaction force exerted by the rotating telescopic cylinder.
According to another aspect of the invention, a method of boring the ground comprises the steps of driving a casing tube into the ground by a casing tube pusher to prevent the ground from collapsing into a bored hole, placing the aforementioned ground-boring apparatus suspended by a movable crane on the casing tube, fastening the casing tube to the supporting frame unit, and digging the ground within the casing tube while rotating the telescopic cylinder about its vertical axis and extending the telescopic cylinder according to the depth of the bored hole.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments/examples with reference to the accompanying drawings.